Method computeStamp

src/main/java/com/trsst/Crypto.java:183–228

Computes hashcash proof-of-work stamp for the given input and bitstrength. Servers can choose which bitstrength they accept, but we recommend at least 20. The colon ":" is a delimiter in hashcash so we replace all occurances in a token with ".". This machine is calculating stamps at a mean rate of

(int bitstrength, long timestamp,
            String token)

     * @return
     */
    public static final String computeStamp(int bitstrength, long timestamp,
            String token) {
        try {
            if (token.indexOf(':') != -1) {
                token = token.replace(":", ".");
            }
            String formattedDate = new SimpleDateFormat("YYMMdd")
                    .format(new Date(timestamp));
            String prefix = "1:" + Integer.toString(bitstrength) + ":"
                    + formattedDate + ":" + token + "::"
                    + Long.toHexString(timestamp) + ":";
            int masklength = bitstrength / 8;
            byte[] prefixBytes = prefix.getBytes("UTF-8");
            MessageDigest sha1 = MessageDigest.getInstance("SHA-1");

            int i;
            int b;
            byte[] hash;
            long counter = 0;
            while (true) {
                sha1.update(prefixBytes);
                sha1.update(Long.toHexString(counter).getBytes());
                hash = sha1.digest(); // 20 bytes long
                for (i = 0; i < 20; i++) {
                    b = (i < masklength) ? 0 : 255 >> (bitstrength % 8);
                    if (b != (b | hash[i])) {
                        // no match; keep trying
                        break;
                    }
                    if (i == masklength) {
                        // we're a match: return the stamp
                        // System.out.println(Common.toHex(hash));
                        return prefix + Long.toHexString(counter);
                    }
                }
                counter++;
                // keep going forever until we find it
            }
        } catch (UnsupportedEncodingException e) {
            log.error("No string encoding found: ", e);
        } catch (NoSuchAlgorithmException e) {
            log.error("No hash algorithm found: ", e);
        }
        log.error("Exiting without stamp: should never happen");
        return null;
    }

    /**
     * Verifies the specified hashcash proof-of-work stamp for the given